Satellite communication systems with continuous wave (CW) Frequency Division Multiple Access (FDMA) signals in the uplink and CW Frequency Division Multiplexed (FDM) signals in the downlink facilitate the use of inexpensive ground terminals because i) earth terminal transmitted power is determined only by the signals at that terminal, ii) modems do not require buffers for burst mode operation, and iii) there is no need for network synchronization. The ability to transfer signals from uplink to downlink beams will be hereafter referred to as interconnectivity.
Full interconnectivity in a conventional FDMA/FDM satellite system requires N.sub.2 transponder channels on board the satellite, where N is the number of non-overlapping beams. Each transponder channel originates at one of the output ports of the on-board demultiplexer and ends at the input of one of the on-board power amplifiers. FDMA/FDM connectivity can be achieved by using time domain bandpass filters which perform the following functions: i) time domain Fourier transformation of the input signals; ii) gating of the unwanted signals; and iii) inverse Fourier transformation of the gated signals (For example see: J. D. Maines, G. L. Moule, C. O. Newton and E. G. S. Paige, "A Novel SAW Variable Frequency Filter" Proc. of IEEE Ultrasonics Symposium, pp. 355-358, 1975 and P. M. Bakken, K. Grythe and A. Ronnekleiv, "The On-board FROBE SAW/Digital Signal Processor", Proc. of the 8th Intern. Conf. Digital Sat. Comms. (ICDSC-8), pp. 617-624, 24-28 April 1989, Guadeloupe). However, the use of time domain band pass filters does not change the nature of the interconnectivity, which remains in the frequency domain, and hence does not help in reducing the complexity of the on-board FDMA/FDM interconnection hardware, which still requires N.sup.2 transponder channels to fully interconnect N beams. Thus, in a conventional FDMA/FDM system having a large number of beams, the required number of transponder channels is large and, consequently, the communications payload hardware on-board the satellite is necessarily complex.
To overcome this difficulty, some conventional systems use burst signals with Satellite Switched Time Division Multiple Access (SSTDMA). Such systems require only N transponder channels for full interconnectivity of N beams since interconnectivity is accomplished by routing signals at different time slots within the same transponder channel to the downlink beams. Full interconnectivity is achieved by providing at least N transponder channels, each channel having at least N time slots directed respectively to the N downlink beams.
In an SSTDMA system, adaptive allocation of capacity is carried out by changing the duration and location of the time slots. Because previously proposed FDMA/FDM multibeam satellite systems allocate capacity in the frequency domain, fully flexible capacity allocation can only be achieved by using techniques such as Variable Bandwidth Variable Center Frequency (VBVCF) demultiplexing and multiplexing (See De Santis U.S. Pat. No. 4,858,225).
Hence, it is an object of the present invention to provide a technique for on-board satellite switching incorporating desirable features of both FDMA and SSTDMA systems, such that earth terminals in a satellite system transmit and receive FDMA/FDM signals in a continuous fashion without knowing that uplink to downlink interconnectivity is provided on-board via Satellite Switched Time Division Multiplexed (SSTDM) periodic burst signals.
It is a further object of the present invention to provide a technique for on-board satellite switching of FDMA/FDM signals which is modulation transparent and allows fully flexible interconnectivity utilizing a reduced number of transponder channels.
It is a still further object of the present invention to provide a technique for on-board satellite switching of FDMA/FDM signals using analog processors operating at Radio Frequencies (RF).